Light emitting device

ABSTRACT

The present disclosure provides a light emitting device, including a serially- connected LED array including a plurality of LED cells on a substrate, including a first LED cell, a second LED cell, and a serially-connected LED sub-array intervening the first and second LED cell; a trench between two neighboring sides of the first and the second LED cells; and a protecting structure formed near the trench to prevent the light-emitting device from being damaged by a surge voltage higher than a normal operating voltage. The protecting structure includes a first insulating layer and a second insulating layer formed over the first insulating layer and partially filling in the trench, wherein two ends of the first insulating layer extending outward from the second insulating layer partially covers the top surfaces of the first and the second LED cells.

REFERENCE TO RELATED APPLICATION

This application claims the right of priority based on U.S. provisionalapplication Ser. No. 61/378,191, filed on Aug. 30, 2010, and the contentof which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to a light emitting device, and more particularlyto a light emitting device with the ability of electrostatic discharge(ESD) protection.

DESCRIPTION OF BACKGROUND ART

FIG. 1 shows an illustration of a light emitting device 10. The lightemitting device 10 comprises a plurality of LED cells 11 (A, B, C, C1,C2, C3) connecting in series by conducting metals 13 on a singlesubstrate 15, wherein each LED cell 11 comprises a first semiconductorlayer 17 on the substrate 15, a second semiconductor layer 19 on thefirst semiconductor layer 17, an active layer 47 (not shown in FIG. 1)arranged between the first semiconductor layer 17 and the secondsemiconductor layer 19, and a conducting metal 13 arranged on the secondsemiconductor layer 19. When one polarity of the AC input passes fromconducting region α to conducting region β, the current flows throughthe LED cells 11 in the following order: A→C1→C2→C3→C→B. The largestpotential difference of the LED cells 11 occurs between LED cells A andB. As shown in FIG. 1, the serially-connected LED array furthercomprises a serially-connected sub-array with four LED cells 11 (C1, C2,C3, C) intervening the terminal LED cells A and B in the seriesconnection.

As shown in FIG. 1, LED A and LED B further comprise a first side (A1,B1) and a second side (A2, B2), respectively. The first sides (A1, B1)of LED A and LED B neighbor to the sub-array, and the second sides (A2,B2) of LED A and LED B neighbor to each other. Besides, a trench T isformed between LED A and LED B. Namely, the trench T is formed betweenthe second sides of LED A and LED B.

Normally, the forward voltage for one LED cell 11 is about 3.5 volt, sothe voltage difference between LED cells A and B should be about3.5*6=21 volts under normal working situation. Because the distancebetween LED cells A and B is very short (about 10˜100 μm), the electricfield strength (E=V/D, V=potential difference, D=distance) between LEDcells A and B is high.

Besides, if there is suddenly a strong electrostatic field from theoutside environment (such as from the human body or the working machine)injecting into the conducting region α, an ultra-high electrical voltageis further inputting to LED cell A, and causes the largest potentialdifference between LED cells A and B. When the value of the electricfield strength reaches a certain value by the strong electrostatic fieldfrom the outside environment, the mediums (air, glue, or otherdielectric materials) therebetween may be ionized, and parts of LEDcells A and B within the electrical field strength are damaged (thedamage region 12) by discharging, which is called the ESD (electrostaticdischarge) damage. The SEM picture of the ESD damage situation is shownin FIG. 2, wherein the ordinary current flow 14 flows in the directionas the arrows indicated in the figure.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a light emitting device, including aserially-connected LED array including a plurality of LED cells on asingle substrate, including a first LED cell, a second LED cell, and aserially-connected LED sub-array including at least three LED cellsintervening the first and second LED cell, wherein each of the first andsecond LED cell including a first side and a second side that the firstside of the first LED cell and/or the second LED cell neighboring to theLED sub-array, and the second side of the first LED cell neighboring tothe second side of the second LED cell; a trench between the secondsides of the first and second LED cells; and a protecting structureformed near the trench to prevent the light emitting device from beingdamaged near the trench by a surge voltage higher than a normaloperating voltage of the light emitting device. The protecting structurefurther includes a first insulating layer filled in the trench and asecond insulating layer formed over the first insulating layer and atleast partially filling in the trench, wherein the first LED cell andthe second LED cell each includes a top surface, and the secondinsulating layer includes an outer surface having two sides covers apart of the top surfaces of the first LED cell and the second LED celland two ends of the first insulating layer extending outward from thesecond insulating layer partially covers the top surfaces of the firstLED cell and the second LED cell, wherein the two ends of the firstinsulating layer is exposed by the two sides of the outer surface of thesecond insulating layer respectively.

A light emitting device in accordance with another embodiment of thedisclosure, wherein each LED cell includes an active layer arrangedbetween a first semiconductor layer and a second semiconductor layer anda conducting metal arranged on the second semiconductor layer of eachLED cell.

A light emitting device in accordance with another embodiment of thedisclosure, wherein the thickness of the protecting structure betweenthe first LED cell and the second LED cell is larger than the sum of thethickness of the first semiconductor layer, the active layer, the secondsemiconductor layer, and the conducting metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of a conventional light emittingdevice;

FIG. 2 illustrates an SEM picture of a light emitting device inaccordance with an embodiment of the disclosure;

FIG. 3 illustrates a light emitting device in accordance with anembodiment of the disclosure;

FIG. 4 illustrates a light emitting device in accordance with anembodiment of the disclosure;

FIG. 5 illustrates a light emitting device in accordance with anembodiment of the disclosure;

FIG. 6 illustrates a cross-section of a light emitting device along line6-6′ in FIG. 5 in accordance with an embodiment of the disclosure;

FIG. 7 illustrates an electric circuit of a light emitting device inaccordance with an embodiment of the disclosure;

FIG. 8 illustrates an electric circuit of a light emitting device inaccordance with an embodiment of the disclosure;

FIG. 9 illustrates a cross-section of a light emitting device inaccordance with an embodiment of the disclosure;

FIG. 10A illustrates a light emitting device in accordance with anembodiment of the disclosure;

FIG. 10B illustrates a cross-section of a light emitting device inaccordance with an embodiment of the disclosure;

FIG. 11 illustrates a light emitting device in accordance with anembodiment of the disclosure;

FIG. 12 illustrates a light emitting device in accordance with anembodiment of the disclosure;

FIG. 13 illustrates light emitting devices in accordance with anembodiment of the disclosure;

FIG. 14 illustrates a light emitting device in accordance with anembodiment of the disclosure;

FIG. 15 illustrates a light emitting device in accordance with anembodiment of the disclosure;

FIG. 16 illustrates a cross-section of a light emitting device inaccordance with an embodiment of the disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments are described hereinafter in accompany with drawings. Tobetter and concisely explain the disclosure, the same name or the samereference number given or appeared in different paragraphs or figuresalong the specification should has the same or equivalent meanings whileit is once defined anywhere of the disclosure.

In order to solve the ESD damage problem, FIG. 3 shows a light emittingdevice 20 in accordance with one embodiment of the present disclosure.The light emitting device 20 comprises a plurality of LED cells 11 (A,B, C, C1, C2, C3) connecting in series by conducting metals 13 on asingle substrate 15, wherein each LED cell 11 comprises a firstsemiconductor layer 17 on the substrate 15, a second semiconductor layer19 on the first semiconductor layer 17, an active layer 47 (not shown inFIG. 3) arranged between the first semiconductor layer 17 and the secondsemiconductor layer 19, and a conducting metal 13 arranged on the secondsemiconductor layer 19. As can be seen in FIG. 3, each conducting metal13 further comprises the extending part with at least two divided metallines. The number of the divided metal lines extended from eachextending part is not limited to what is disclosed herein. In order tohave the LED array connected in series, the first semiconductor layer 17of LED A is electrically connected to the second semiconductor layer 19of the adjacent LED cell, for example, LED C1. When one polarity of theAC input passes from conducting region α to conducting region β, thecurrent flows through the LED cells 11 in the following order:A→C1→C2→C3→C→B. The largest potential difference of the LED cells 11occurs between LED cells A and B. As shown in FIG. 3, theserially-connected LED array further comprises a serially-connectedsub-array with four LED cells 11 (C1, C2, C3, C) intervening theterminal LED cells A and B in the series connection.

As shown in FIG. 3, LED A and LED B further comprise a first side (A1,B1) and a second side (A2, B2), respectively. The first sides (A1, B1)of LED A and LED B neighbor to the sub-array and the second sides (A2,B2) of LED A and LED B neighbor to each other. Besides, a trench T isformed between LED A and LED B. Namely, the trench T is formed betweenthe second sides of LED A and LED B.

To prevent the ESD damage, a protecting structure is formed near thetrench T to prevent the light emitting device from being damaged at aregion near the trench by a surge voltage higher than a normal operatingvoltage. In this embodiment, a first insulating layer 23 is formedbetween the LED cells, and a second insulating layer 21 is furtherformed over the first insulating layer 23 in the region between two LEDcells 11 with high electrical field strength, for example, the trench T.The second insulating layer 21 can be optionally thicker than the firstinsulating layer 23. Taking the light emitting device 20 in FIG. 3 forinstance, LED A and LED B are electrically connected in series with four(more than three) LED cells 11 connected in-between and therefore suffera high electric field strength over a certain value, and therefore thesecond insulating layer 21 is formed to cover the part of the topsurfaces of the first insulating layer 23 and part of the top surfacesof LED A and LED B to isolate the LED cells 11 from the ESD damage.Besides, without the first insulating layer 23, the second insulatinglayer 21 only can also be the protecting structure to cover the regionbetween two LED cells 11 with high electrical field strength, forexample, the exposed surfaces of the substrate 15, the side surface ofthe first semiconductor layer 17, and the side surface of the secondsemiconductor layer 19 between LED cells A and B. Besides, the materialsof the insulating layer 21 and/or 23 can be insulating materials such asAlO_(x1), SiO_(x2), SiN_(x3), and so on, and the insulating layer 21and/or 23 may be a composite structure with multi layers formed bydifferent materials. For example, the second insulating layer 21 may beformed by the combination of one layer of SiO_(x4) with the thickness of2100 Å and one layer of AlO_(x5) with the thickness of 2000 Å, and thefirst insulating layer 23 may be formed by only one layer of SiO_(x4)with the thickness of 2100 Å. (The index words X1-X5 here are numbers,which could be integers or decimals, and can be the same or different.)

FIG. 4 shows a light emitting device 30 in accordance with anotherembodiment of the present disclosure. As can be seen, the secondinsulating layer 21 covers most part of the top surfaces of LED A andLED B. For the same reason mentioned above, the insulating layers 21 and23 can also be a composite structure with multi layers formed bydifferent materials or a thick single layer, and the number or thethickness of the second insulating layer 21 on the covered top surfacesof LED A and LED B can be more than those in other regions.

FIG. 5 shows a light emitting device 40 in accordance with anotherembodiment of the present disclosure. In the embodiment, reducing theelectrical field strength is another method to prevent the ESD damage.As shown in FIG. 5, an insulating wall 41 is formed between LED A andLED B (the region with a high electric field strength or between twoadjoining LED cells 11 which are electrically connected in series andwith more than three LED cells 11 connected in-between). Because theinsulating wall 41 is formed by the insulating material, the electriclines originating from LED A cannot be directed to LED B by penetratingthe insulating wall 41 directly and should be extended along thecontours of the insulating wall 41 instead. The length of the electricline is extended, and therefore the electrical field strength (E=V/D,V=potential difference, D=distance) between LED A and LED B is reduced.In order to extend the length of the electric lines between LED A andLED B, the insulating wall 41 located between LED A and LED B should beformed at the shortest route from LED A to LED B to shield the electriclines coming from LED A or LED B. In other words, as shown in FIG. 5,the thickness of the insulating wall 41 in the trench should besubstantially larger than the sum of the thickness of the firstsemiconductor layer 17, the active layer 47, the second semiconductorlayer 19, and the conducting metal 13. Preferably, the thickness of theinsulating wall 41 in the trench should be larger than 1.5 times the sumof the thicknesses of the first semiconductor layer 17, the active layer47, the second semiconductor layer 19, and the conducting metal 13.

FIG. 6 is the cross-section of the 6-6′ line shown in FIG. 5. In FIG. 6,the first insulating layer 23 covers conformably along the side walls ofLED A and LED B (including the first semiconductor layers 17, the secondsemiconductor layers 19, and the active layers 47), part of the topsurfaces of the LED A and LED B, and part of the top surface of thesubstrate in the trench T directly. Besides, the insulating wall 41 canbe formed on the first insulating layer 23 and is higher than the LED Aand LED B, therefore, the electric lines from LED A to LED B can beshielded by the insulating wall 41. However, the exact position of theinsulating wall 41 can be modified and should not be limited. Forexample, the insulating wall 41 can also be formed on the top surface ofthe substrate 15 directly, or the insulating wall 41 can have a specificpattern formed by the traditional CVD method and the photolithographymethod.

FIG. 7 shows an electric circuit of a light emitting device 50 inaccordance with another embodiment of the present disclosure. As theexperimental result indicates, a floating conductive line 55electrically connecting to the LED cell 11 with the electric potentiallevel between the highest potential and the lowest potential and locatedbetween the LED cell 11 with the highest potential and the LED cell 11with the lowest potential can reduce the ESD damage. As shown in FIG. 7,a floating conductive line 55 can connect to the conducting metal 13between LED C2 and LED C3.

Similar to FIG. 7, FIG. 8 is an electric circuit of a light emittingdevice 60 in accordance with another embodiment of the presentdisclosure. Rather than forming a floating conductive line 55 connectingto the conducting metal 13, one grounding conductive line 65 is formedbetween the LED cell 11 with the highest potential and the LED cell 11with the lowest potential, and the grounding conductive line 65 isgrounded by connecting to outside.

FIG. 9 is the cross-section of the embodiment shown in FIG. 7 and FIG.8. The floating (grounding) conductive line 55(65) can be formed on theinsulating layer 23 or on the substrate 15 directly between two LEDcells 11.

FIG. 10A discloses a light emitting device 70 in accordance with anotherembodiment of the present disclosure. Since the ESD damage normallycauses the failure of the LED and is not easy to be avoided, additionalESD damage regions 75 are formed to confine the ESD damage happened inthe specific regions. As shown in FIG. 10A, there are two additional ESDdamage regions 75 extending from the conductive metals 13. Because thetwo additional ESD damage regions 75 are facing to each other closely,the higher electric field strength is caused therebetween, and the ESDdamage may happen between the additional ESD damage regions 75 moreeasily. The additional ESD damage regions 75 are two additional metalplates that do not function with the LED cells 11, therefore, they canhelp to maintain the working function of the LED cells 11. Besides, theedge of the additional ESD damage region 75 facing the other one can beroughened to form tips, which raises the probability of the ESDphenomenon happening in the predetermined regions.

FIG. 10B is the cross-section of the embodiment shown in FIG. 10A. Thetwo additional ESD damage regions 75 are formed on the insulating layer23 or on the substrate 15 directly between two LED cells 11.

FIG. 11 shows a light emitting device 80 in accordance with anotherembodiment of the present disclosure. Because the ESD damage comes fromthe high electric field strength, and the electric field strengthbetween two objects depends on the potential difference and the distanceof the two objects. As shown in FIG. 11, indicated by the experimentalresult, the distance (D) between two adjoining LED cells 11 with morethan three LED cells 11 connected in-between should be larger than 15μm. Preferably, the distance (D) between two adjoining LED cells 11 withmore than three LED cells 11 connected therein should be more than 30μm. The distance (D) here is identified as the shortest distance betweentwo first semiconductor layers 17 of two adjoining LED cells 11. Inaddition, the “adjoining LED cells” here means any two LED cells 11 withthe shortest distance from the first semiconductor layers 17 of the LEDcell to the first semiconductor layer of the other LED cell, wherein thedistance (D) is preferred to be smaller than 50 μm.

FIG. 12 shows a light emitting device 90 in accordance with anotherembodiment of the present disclosure. Similar to the embodiment shown inFIG. 11, to prevent the ESD damage happened between the conductingmetals 13 of the two adjoining LED cells 11, the distance (d) of theconducting metals 13 of the two adjoining LED cells 11 should be morethan 100 μm. This design is suitable for the two adjoining LED cells 11with large potential difference therebetween and/or with more than threeLED cells 11 connected in-between. Preferably, the distance (d) of theconducting metals 13 of the two adjoining LED cells 11 should be morethan 80 μm. The “distance of the conducting metals” here is defined asthe shortest distance from the conducting metal 13 of the LED cell 11 tothe conducting metal 13 of the adjoining LED cell. In addition, the“adjoining light-emitting diode cells” here means any two LED cells 11with the shortest distance from the first semiconductor layers 17 of theLED cell 11 to the first semiconductor layer 17 of the other LED cell11, wherein the distance (d) is preferred to be smaller than 50 μm.

As shown in FIG. 13, the high electric field strength often occursbetween two adjoining LED cells 11 with more than three LED cells 11connected in-between (with large potential difference), such as LEDcells A and B shown in FIG. 12. In order to prevent the LED cells 11with large potential difference from being too close, when a series ofLED cells 11 formed on a substrate 15, the series of LED cells 11 shouldchange its arranging direction when certain amount of LED cells 11 arealigned with one direction, for example, which more than three LED cells11 aligned with. In other words, the arranging direction of the seriesof the LED cells 11 should be changed often to avoid any two LED cells11 with large potential difference or with more than three LED cells 11connected in-between located adjoining to each other. FIG. 13 showsthree diagrams of different arrange configurations of a series LED cells11 on a single substrate 15 in accordance with embodiments of thepresent disclosure. In each diagram, a series of the LED cells 11disposed (formed by epitaxy or attached to the substrate by metalbonding or glue bonding) on a single substrate 15 with the bonding pads105 formed at two ends of the series of the LED cells. The arrow hereindicates the extending direction 103 (the connection order) of the LEDcells 11. In each arrangement, any two of the adjoining LED cells do nothave the large potential difference therebetween. In detail, as shown inFIG. 13, each serially-connected LED array comprises at least eight LEDcells and at least more than two branches. In order not to let any twoof the adjoining LED cells have too large potential differencetherebetween, each LED array shown in the figure changes its arrangingdirection with every two successive LED cells.

FIG. 14 shows a light emitting device 110 in accordance with anotherembodiment of the present disclosure. According to the surface electriccharge distribution of the objects, the surface electric dischargedensity per unit area of the object is large when the object has a smallcurvature radius. Another cause of the damage of the LED cells 11 called“point discharge” often happens at the position with high surfaceelectric discharge density per unit area. Therefore, to prevent the“point discharge” phenomenon, the contours of the LED cells 11 aremodified in this embodiment. As shown in FIG. 14, the upper corners ofthe first semiconductor layer 17 between LED cells A and B arepatterned, for example, rounded. The above modification is not limitedto the identified positions, and all of the corners of the LED cells 11can be patterned to be rounded. Furthermore, not only the firstsemiconductor layer 17 but the second semiconductor layer 19 can also berounded, especially for the edges of the LED cell close to the secondside, which has the smaller distance from the adjoining LED cell.Preferably, the radius of the curvature of the patterned corner is notless than 15 μm.

With the similar concept of the embodiment disclosed in FIG. 14, toprevent the “point discharge” phenomenon, FIG. 15 shows that theterminal 123 of each divided metal line of the LED cells 11 can bepatterned by forming the round metal plates 123. The shape of theterminal metal 123 is not limited to the round shape. As indicated bythe experimental result, any shape of the terminal metal 123 formed atthe terminal with the enlarged portion or the radius larger than theline width of the conducting metal 13 can be formed to reduce the “pointdischarge” damage.

FIG. 16 shows a cross-section of a light emitting device 130 inaccordance with another embodiment of the present disclosure. In orderto reduce the undesirable discharge, a smoother path facilitating thecurrent flow can help. As shown in FIG. 16, in order to let the currentspread from the conducting metal widely, a current blocking layer 133 isprovided beneath the conducting metal 13. The current blocking layer 133is made by a dielectric material which is an insulator, such as SiO_(y1)or SiN_(y2). However, the current blocking layer also blocks most pathsthat the current flows. If the current cannot disperse along the normalpath, it leaks out in other forms such as ESD or point discharge.Therefore, in this embodiment, the current blocking layer 133 is formedbeneath the conducting metal 13, not under the terminal of theconducting metal 13. This design let the current at the terminal of theconducting metal 133, where the discharge is caused most easily, to flowmore smoothly and reduce the probability the current leaking out alongthe discharge path. Besides, a transparent conducting layer 135 such asITO, IZO, ZnO, AZO, thin metal layer, or the combination thereof canalso be optionally formed on the second semiconductor layer to help thecurrent spreading.

The embodiments mentioned above are used to describe the technicalthinking and the characteristic of the invention and to make the personwith ordinary skill in the art to realize the content of the inventionand to practice, which could not be used to limit the claim scope of thepresent invention. That is, any modification or variation according tothe spirit of the present invention should also be covered in the claimscope of the present disclosure. For example, the electric connectingmethod is not limited to the serial connection. The ESD protectionmethods shown as the embodiments above can be applied to any twoadjoining light emitting diode cells with a high electric field strengthover a certain value or with more than three LED cells connectedin-between electrically connect in parallel or in the combination ofserial and parallel.

What is claimed is:
 1. A light emitting device, comprising: aserially-connected LED array comprising a plurality of LED cells on asubstrate, comprising: a first LED cell, a second LED cell, and aserially-connected LED sub-array comprising at least three LED cellsintervening the first and second LED cell, wherein each of the first andsecond LED cell comprising a first side and a second side, the firstside of the first LED cell and/or the second LED cell neighboring to theLED sub-array, and the second side of the first LED cell neighboring tothe second side of the second LED cell; a trench between the secondsides of the first and second LED cells; and a protecting structureformed near the trench to prevent the light emitting device from beingdamaged near the trench by a surge voltage higher than a normaloperating voltage of the light emitting device, wherein the protectingstructure further comprises: a first insulating layer filled in thetrench; and a second insulating layer formed over the first insulatinglayer and at least partially filling in the trench, wherein the firstLED cell and the second LED cell each comprises a top surface, and thesecond insulating layer has an outer surface having two sides covers apart of the top surfaces of the first LED cell and the second LED celland two ends of the first insulating layer extending outward from thesecond insulating layer and partially covers the top surfaces of thefirst LED cell and the second LED cell, wherein the two ends of thefirst insulating layer is exposed by the two sides of the outer surfaceof the second insulating layer respectively.
 2. The light emittingdevice as claimed in claim 1, wherein the first LED cell and the secondLED cell each comprises a side wall near the second side, and the firstinsulating layer covers conformably on the side walls of the first LEDcell and the second LED cell.
 3. The light emitting device as claimed inclaim 1, wherein the second insulating layer comprises multiple layers.4. The light emitting device as claimed in claim 1, wherein each LEDcell comprises an active layer arranged between a first semiconductorlayer and a second semiconductor layer and a conducting metal arrangedon the second semiconductor layer.
 5. The light emitting device asclaimed in claim 4, wherein the thickness of the protecting structure inthe trench is larger than the sum of the thickness of the firstsemiconductor layer, the active layer, the second semiconductor layer,and the conducting metal.
 6. The light emitting device as claimed inclaim 4, wherein the thickness of the protecting structure in the trenchis larger than 1.5 times the sum of the thickness of the firstsemiconductor layer, the active layer, the second semiconductor layer,and the conducting metal.
 7. The light emitting device as claimed inclaim 4, wherein the distance between the first semiconductor layer ofthe first LED cell and the first semiconductor layer of the second LEDcell is smaller than 50 μm.
 8. The light emitting device as claimed inclaim 7, wherein the distance between the first semiconductor layer ofthe first LED cell and the first semiconductor layer of the second LEDcell is larger than 15 μm.
 9. The light emitting device as claimed inclaim 7, wherein the distance between the first semiconductor layer ofthe first LED cell and the first semiconductor layer of the second LEDcell is larger than 30 μm.
 10. The light emitting device as claimed inclaim 5, wherein the minimum distance between the conducting metal ofthe first LED cell and the conducting metal of the second LED cell islarger than 80 μm.
 11. The light emitting device as claimed in claim 5,wherein the minimum distance between the conducting metal of the firstLED cell and the conducting metal of the second LED cell is larger than100 μm.
 12. The light emitting device as claimed in claim 4, wherein theconducting metal further comprises an extending part with at least onefinger and the at least one finger comprises an enlarged terminal. 13.The light emitting device as claimed in claim 4, wherein one of the LEDcells further comprises a current blocking layer formed beneath theconducting metal.
 14. The light emitting device as claimed in claim 4,wherein each of the LED cells further comprises a transparent conductinglayer formed on the second semiconductor layer.
 15. The light emittingdevice as claimed in claim 1, wherein the serially-connected LED arraycomprises at least eight LED cells with two branches and the LED arraychanges its arranging direction every two successive LED cells.
 16. Thelight emitting device as claimed in claim 4, wherein each of the firstsemiconductor layers of the LED cells comprises a round corner with aradius of the curvature not less than 15 μm.
 17. The light emittingdevice as claimed in claim 16, wherein the round corner is adjacent tothe second side.
 18. The light emitting device as claimed in claim 4,wherein the first semiconductor layer of one LED cell in theserially-connected LED array is electrically connected to the secondsemiconductor layer of the adjacent LED cell in the serially-connectedLED array.
 19. The light emitting device as claimed in claim 1, thesubstrate comprises a top surface, wherein the first insulating layer inthe trench directly contact the top surface of the substrate.
 20. Thelight emitting device as claimed in claim 1, wherein the substrate is asingle crystalline substrate, and the LED cells of the series LED arrayare epitaxially grown on the single substrate.
 21. The light emittingdevice as claimed in claim 1, wherein the serially-connected LED arrayfurther comprises two bonding pads arranged at corners of the substrate.